The present invention relates to a peak level detecting apparatus, and more particularly to a peak level detecting apparatus for image sensors by which the maximum light value of a remote object is detected effectively utilizing the dynamic range of image sensors.
In recent years, technology utilizing a photoelectric device for an image sensor has been developed for optical devices such as for cameras. For example, a charge coupled device has been employed as an automated range finding apparatus or automated exposure controller apparatus for still cameras. The automated range finding apparatus comprise a couple of line image sensors (which are made up of plural photoelectric devices such as photo diodes linearly aligned) arranged at different distances from an optical axis to output signal charges which are induced in photo-electric devices through a charge coupled devices (CCD). Each signal charge corresponds to a specific part of the optical image of the remote object, therefore, by correlating the patterns of signal charges from these couple of line image sensors, the phase difference of these patterns makes it possible to calculate the distance to the remote object.
For the precise detection of the phase difference, it is required to receive signals from the remote object within the dynamic range of the line image sensors and CCD. Heretofore, because these devices are saturated over the dynamic range by receiving too great an incident signal, a part of the pattern from the signal is clipped. Consequently, the correlation measurement is accomplished for patterns different from the actual ones. Due to this fact, the phase difference measurement result contains error and precision range finding could not be achieved.
To improve the accuracy of the phase difference measurement, heretofore, it was proposed that the exposure process be stopped when the average of each charge reaches a predetermined threshold level to attain charge patterns without clipping for precise range finding by detection of the signal charge amount induced in each photo electric device.
In FIG. 7, one example of a phase difference detector for a range finding apparatus of the aforementioned prior art method is illustrated. The detector comprises lens assembly (1), image forming means (2) which is arranged at the back of the lens assembly (1), condensing lens (3), separating lens (4) and phase a difference detector which are arranged at the back of the image forming means (2) in the above-mentioned order parallel to the light axis of the apparatus. The aforesaid phase difference detector comprises a couple of line image sensors (5), (6) by which the two images made by the separator lens (4) are photoelectrically transformed, and a circuit (7) by which a signal processing is performed to judge the accuracy of focusing from signal charges induced in the aforesaid line image sensors (5) and (6) corresponding to the distribution of the light intensity.
Hereby, the images on the aforesaid line image sensors from the aforesaid separating lens come closer to the light axis (8) when the focused image of the remote object is located in front of the aforesaid image forming means, and they depart farther from the light axis (8) when the focused image of the remote object is located behind the image forming means. When the range finding is accurate, the focused image is located in the predetermined specific portion away from the light axis. Due to this fact, the range finding process is performed by the measurement of the distance of the image from the light axis (8) by the signal induced in the line image devices (5), (6).
To detect a relative position on the line image sensors (5) and (6), a method of phase difference detection is employed. This is obtained by a calculation of a correlation value of the formula (1) for two images on line image sensors (5) and (6), and judgment for the focusing accuracy is done through a detection based on the relative amount of shift (phase difference) of the two images while the correlation is minimized. EQU H(L)=B(K) - R(K-L-1) (1)
In equation (1), L is, for example, an integer variable between 1 and 9, which corresponds to the aforesaid relative amount of shift.
B(K) is, for example, a serial output signal from each element of one of the line image sensors (5).
R (K-L-1) is a serial output signal from each element of the other line image sensors (6). Nine correlations H(1), H(2), . . . H(9) are obtained from the aforesaid formula (1) when L is changed from 1 to 9. For example, in case it is predetermined that the image is correctly focused on the image forming means when H(5) is minimized, the phase difference between L=5 and other values which give the minimized H on the measurement is obtained as the actual value to correct focusing.
FIG. 8 is a block diagram of an example of the processing circuit in FIG. 7. In this example, the aforesaid processing circuit (7), line image sensors (5) and (6) are formed on a semiconductor chip module.
In FIG. 8, elements (5) and (6) are line image sensors, and predetermined numbers of photo diode cells (bl) .about.(bm), (rl) .about.(rn) are arranged in line at opposite positions for the light axis (8). A circuit is arranged next to the aforesaid line image sensors to transfer the charges induced in the image sensors. Herein, the elements (7) and (8) are CCD charge storage devices which comprise charge storage elements T(bl) .about.T(bm), T(rl) .about.T(rn) corresponding to each photo diode X(bl) .about.X(bm), X(rl) .about.X(rn). The elements (9) and (10) are CCD charge transferring devices which comprise charge transferring elements C(bl) .about.C(bm), C(rl) .about.C(rn) corresponding to each charge storage element T(bl) .about.T(bm), T(rl) .about.T(rn). When transferring gates (11) and (12) are turned "ON" by a gate control signal TG, charges which are stored in each storage element T(bl) .about.T(bm), T(rl) .about.T(rn) in the charge storage means (7) and (8) are transferred in parallel to the charge transferring elements C(bl) .about.C(bm), .about.C(rl) C(rn). The elements (13) and (14) are floating gates which generate voltages corresponding to the amount of signal charge induced in the photo diodes X(bl) .about.X(bm), X(rl) .about.X(rn). Herein, because the floating gates (13) and (14) are deposited on the charge transferring path with electric field-coupling (non contactedly) between the line image sensors (5) and the charge storage means (7), and the line image sensors (6) and the charge storage means (8), respectively the amount of induced signal charges can be read out without destruction.
The elements (15) and (16) are multiplexers for parallel-serial transformation of the parallel outputted signals from the charge transferring elements C(bl) .about.C(bm), C(rl) .about.C(rn) to serial signals S(b) and S(r). The element (17) is a circuit for calculating the correlation between the signals S(b) and S(r) and outputting a result H(L) of the aforesaid formula (1).
The element (18) is a circuit which generates control signals required for the overall operation such as a gate electrode signal T.PHI. which is applied to the so-called charge transferring gate of the charge storage means (7) and (8), a gate control signal TG which operates the "ON" and "OFF" status of the transferring gates (11) and (12), a gate electrode signal C.PHI. which is applied to the so-called charge transferring gate of the charge storage means (9) and (10) and channel switching signals CH(b) and CH(r) for multiplexers (15) and (16).
In the figure, the charge detecting circuit (19), which is indicated within the dotted line, comprises an impedance transforming amplifier (20) connected to the floating gate (13) to receive the signal therefrom, a resetting transistor (21) which is connected between an input terminal of the aforesaid amplifier and an electric voltage source V(DD), and a comparator (22) to which is applied a reference voltage V(ref) at an inverting input terminal and an output signal S(a) from the amplifier (20) at a non inverting input terminal. Comparator (22) outputs a detecting signal D(f) when the voltage of signal S(a) corresponding to a signal S(f) becomes higher than the reference voltage V(ref). When the detection signal D(f) is supplied to the control signal generating circuit (18), it stops the exposure process for the range finding operation by outputting a gate control signal T.phi. by which the transferring gate TG is closed.
In this context, the floating gate (13) stops the exposure process to prevent an occurrence of overflow of signal charges and to maintain the precise measurement for the correlation H(L), as it is determined that a permissible amount for charge capacity is obtained for each element of the charge storage means (7), (8) and charge transferring means (9), (10) when signal S(a), which represents an average of the signal charges induced in the photo diodes X(bl) .about.X(bm) (which are the same as the charges induced in the photo diodes X(rl) .about.X(rn)), reaches the predetermined voltage level V(ref). Hereby, the exposure process for range finding is begun by clamping the signal S(f) to the voltage source V(dd) by turning "ON" a transistor (21) temporarily. As the signal S(f) changes according to the amount of light received, the time length for exposure can be determined according to the amount of light incident from the remote object without occurrence of saturation.
However, though the aforementioned improvements in the prior art are intended to ensure dynamic range with control of the amount of light received through a calculation of the average value of charge signal induced in all photo diodes, they could not successfully prevent the occurrence of signal saturation in the case when there are higher brightness portions in the remote object compared to the background of the object. In such case, the signal corresponding to the aforesaid higher brightness portion could not help but saturate the measurement based on the average value of all the photo diodes. Further, in the case where one portion of the remote object is much brighter with a dark background, even though some photo diodes have saturated with over-induced charges, the average signal S(a) tends to require a long exposure time to reach the reference level V(ref). This causes an increase of dark current at the sensors in exposure time, which makes the signal to noise ratio worse.